This is a application to identify cellular and viral factors that directly interact with Vif. Initially, three overlapping or complementary approaches will be taken to this end. These include association of proteins with GST-Vif fusion proteins, isolation of cDNAs that encode proteins that interact with Vif in a yeast two hybrid screen, and coimmunoprecipitation and crosslinking studies. Candidate proteins will be examined further by screening with many Vif mutants to find mutants that reduce or eliminate binding or association of the two proteins. Such mutations will then examined in depth for their effect on replication and specific properties of Vif.